Strategy Guideline: Energy 

Retrofits for Low-Rise Multifamily 

Buildings in Cold Climates

K. Brozyna

IBACOS, Inc.

L. Badger

Vermont Energy Investment Corporation

April 2013
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iii

Strategy Guideline: Energy Retrofits for Low-Rise 

Multifamily Buildings in Cold Climates

Prepared for: 

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program 

Office of Energy Efficiency and Renewable Energy 

15013 Denver West Parkway 

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

K. Brozyna 

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, PA 15222

L. Badger

Vermont Energy Investment Corporation

128 Lakeside Avenue, Suite 401

Burlington, VT 05401

NREL Technical Monitor: Michael Gestwick

Prepared under Subcontract No. KNDJ-0-40341-00 

April 2013
iv

Acknowledgments

The authors thank the U.S. Department of Energy’s Building America program for funding this 

research. They also wish to thank the following staff members of Vermont Energy Investment 

Corporation for their technical expertise in developing this Strategy Guideline: Sam Dent, Mary 

Jane Poynter, Ken Tohinaka, and Sean Bleything.
v 

Contents

Acknowledgments ..................................................................................................................................... iv

List of Figures ............................................................................................................................................ vi

Definitions .................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

1 Introduction ........................................................................................................................................... 1

2 Roof Repair or Replacement ............................................................................................................... 3

2.1 Energy Efficiency Opportunities .........................................................................................3

2.2 Building Science in Energy Efficiency Measures ...............................................................4

2.2.1 Air Seal ....................................................................................................................4

2.2.2 Insulate .....................................................................................................................7

2.3 Risks and Other Considerations ...........................................................................................8

2.4 Estimated Energy Savings ...................................................................................................9

2.4.1 Example 1: 5-Unit Town House Building .............................................................10

2.4.2 Example 2: 31-Unit Double-Loaded Corridor .......................................................10

3 Wall Repair or Gut Rehab .................................................................................................................. 11

3.1 Energy Efficiency Opportunities .......................................................................................11

3.2 Building Science in Energy Efficiency Measures .............................................................12

3.2.1 Air Seal ..................................................................................................................13

3.2.2 Fenestration Replacement or Repair ......................................................................15

3.2.3 Insulate ...................................................................................................................16

3.3 Risks and Other Considerations .........................................................................................17

3.4 Estimated Energy Savings .................................................................................................18

3.4.1 Example 1: 5-Unit Town House Building .............................................................18

3.4.2 Example 2: 31-Unit Double-Loaded Corridor .......................................................18

4 Boiler Maintenance and Retrofit ....................................................................................................... 20

4.1 Energy Efficiency Opportunities .......................................................................................20

4.2 Building Science in Energy Efficiency Measures .............................................................21

4.3 Risks and Other Considerations .........................................................................................23

4.4 Estimated Savings ..............................................................................................................24

5 Retrofit Measure Strategy Guideline Overview ............................................................................... 26

References................................................................................................................................................. 28

Appendix A: Resources for Roof Repair or Replacement.................................................................... 31

Appendix B: Resources for Wall Repair or Gut Rehab......................................................................... 32

Appendix C: Resources for Boiler Maintenance and Retrofit .............................................................. 33
vi 

List of Figures

Figure 1. 5-unit town house (left); 31-unit double-loaded corridor multifamily buildings (right)....... 2

Figure 2. Before (left) and after (right) air sealing penetrations into any unconditioned roof 

cavities................................................................................................................................................... 3

Figure 3. Mold growing on the underside of roof sheathing (left) (DOE 2011); discoloration of 

insulation (right) ................................................................................................................................... 4

Figure 4. The thermal boundary and air barrier separate the unconditioned space from the 

conditioned space of a structure, preventing air infiltration indicated by the arrows 

(DOE 2011)............................................................................................................................................. 5

Figure 5. The air sealing benefits of spray foam make it a great option to insulate around rim 

or band joists ........................................................................................................................................ 6

Figure 6. Mold growth on the inside surface of exterior walls (left) and inside the wall cavity 

(right). Both conditions are likely caused by an improperly air sealed and insulated woodframed wall assembly. ....................................................................................................................... 11

Figure 7. Signs of poor air sealing and insulation performance.......................................................... 12

Figure 8. Poor air sealing around windows (ENERGY STAR 2011c)................................................... 14

Figure 9. (Left) Good air sealing around windows and (right) between drywall and subfloor ......... 15

Figure 10. Proper installation of continuous rigid foam (ENERGY STAR 2011c), showing the 

use of cap nails and proper sealing of seams................................................................................. 17

Figure 11. Typical multifamily heating unit without microprocessor controls (Klein 2011) ............. 20

Figure 12. Replacement boiler with microprocessor controls (left); associated water flow 

control (right) ...................................................................................................................................... 23

Figure 13. Roof repair or replacement retrofit measure ....................................................................... 27

Figure 14. Wall repair or gut rehab retrofit measure ............................................................................. 27

Figure 15. Boiler maintenance and retrofit measure............................................................................. 27

Unless otherwise noted, all figures and photos were created by IBACOS. 
vii 

Definitions

ACH Air changes per hour

ACH50 Air changes per hour at 50 Pascals of pressure, calculated by 

multiplying CFM50 by 60 and dividing by the volume of the 

building 

ACHn Natural air change rate, approximated by modifying the 

ACH50 value with factors to reflect local wind speeds, 

external shielding such as trees, and the height and size of the 

building 

AFUE Annual fuel utilization efficiency

BLPM Brushless permanent magnet (motor)

CFM Cubic feet per minute

CFM50 Cubic feet per minute at 50 Pascals of house pressure 

measured with a blower door 

DOE U.S. Department of Energy

IECC International Energy Conservation Code

IRC International Residential Code

RESNET Residential Energy Services Network

SF Square foot
viii

Executive Summary

This Strategy Guideline explains the benefits of evaluating and identifying energy efficiency 

retrofit measures that could be made during renovation and maintenance of multifamily 

buildings. It focuses on low-rise multifamily structures (three or fewer stories) in a cold climate. 

These benefits lie primarily in reduced energy use, lower operating and maintenance costs, 

improved durability of the structure, and increased occupant comfort. 

This Strategy Guideline focuses on retrofit measures for three typical building maintenance 

categories:

1. Roof repair or replacement

2. Exterior wall repair or gut rehab

3. Heating system maintenance. 

All buildings are assumed to have a flat ceiling and a trussed roof, wood- or steel-framed exterior 

walls, and one or more single or staged boilers. 

When making retrofits in any of these categories, consider extraneous factors such as the age of 

the building occupants (e.g., housing for senior citizens requires higher heating levels), historic 

preservation, and available financial assistance. 

Estimated energy savings realized from the retrofits will vary, depending on the size and 

condition of the building, the extent of efficiency improvements, the efficiency of the heating 

equipment, the cost and type of fuel, and the climate location. 
1 

1 Introduction

This Strategy Guideline highlights the benefits of identifying potential opportunities for energy 

efficiency retrofit measures to be completed at the time of routine building maintenance or nonenergy-related repairs. Routine maintenance, whether for repairs or replacement, provides 

important opportunities to address the energy efficiency of the building and its subsystems. 

Traditionally, energy efficiency improvements are not considered or are overlooked when 

maintenance plans are created and executed. When energy-related improvements are combined 

with maintenance or repair operations, significant savings are captured: reductions in energy use 

are realized, operating and maintenance costs are reduced, and structures become more durable. 

Energy efficiency improvements can also be implemented more cost effectively if they are 

coordinated with concurrent maintenance or repair work than if postponed and addressed under 

an independent scope of work. 

This Strategy Guideline can serve as a reference for building owners, developers, and facilities 

personnel engaged in the routine maintenance of residential multifamily buildings to help them 

develop the thought processes needed to identify opportunities for incorporating energy 

efficiency measures in their work. It may also be useful for initiating discussion and 

collaboration between contractors and building owners. Affordable housing agencies likewise 

may find this Strategy Guideline useful when they are involved in these types of renovations. 

Although the basic principles outlined in this Strategy Guideline apply to most buildings in cold 

climates, the focus is on low-rise multifamily structures (three or fewer stories) in cold and very 

cold climates.1

 This Strategy Guideline addresses energy efficiency improvement opportunities 

and potential savings associated with three typical, non-energy efficiency-related building 

maintenance categories: 

1. Roof replacement or repair 

2. Exterior wall repair or gut rehab

3. Heating system maintenance. 

The basic building component assemblies are assumed to be a flat ceiling and a trussed roof, 

wood- or steel-framed exterior walls, and one or more single or staged boilers. Figure 1 depicts 

the two basic multifamily building types modeled for the energy efficiency savings analysis 

sections.

 

1 Cold and very cold climates typically are defined as regions with more than 5,400 heating degree days (65°F) and 

fewer than 12,600 heating degree days (Building America 2011).
2 

Figure 1. 5-unit town house (left); 31-unit double-loaded corridor multifamily buildings (right) 

(Housing Vermont 2010) 

(Photos courtesy Efficiency Vermont)
3 

2 Roof Repair or Replacement

2.1 Energy Efficiency Opportunities 

When a roof needs repair or replacement, use the opportunity to consider energy efficiency 

measures that may be implemented concurrently with the scheduled repairs. In many cases, roof 

repairs are required because of underlying energy performance problems such as poor insulation 

or air sealing. Taking steps to create an energy efficient roof assembly will reduce operating 

costs, provide increased occupant comfort, improve the durability of the roof, and reduce longterm maintenance costs. Figure 2 shows the same roof before and after air sealing penetrations 

between the conditioned space and any unconditioned roof cavities. The ice dams and icicles 

shown in the left photo are formed when heat from the conditioned space escapes into the attic, 

warming the roof surface and melting the adjacent snow. The resultant meltwater flows down to 

the roof edge, where, if the ambient temperature is below freezing, it refreezes and causes an ice 

dam. By reducing the heat loss through air sealing and insulation, the roof is no longer artificially 

warmed, and ice dam and icicle formation is minimized. 

Figure 2. Before (left) and after (right) air sealing penetrations into any unconditioned roof cavities 

(Housing Vermont 2010) 

(Photos courtesy Efficiency Vermont)

To help guide where and what energy efficiency improvements might be incorporated into roof 

repairs, evaluate the condition of the attic insulation, roofing materials, sheathing, air sealing,

and exterior flashing details (if applicable) to assess what conditions may have contributed to the 

damage. Ask the following questions: 

1. Is mold or rot present in the framing or roof sheathing?

2. Is the insulation discolored? 

3. Is there excessive ice damming in winter?

4. Is there frost on the underside of the roof sheathing in winter? 

5. Is the roofing material damaged, and if so, where? 
4 

Figure 3 shows visual evidence of poor air sealing and insulation performance. The photo on the 

left shows mold growing on the underside of the roof sheathing. This happens when warm, moist 

air condenses on the cold sheathing surface. The photo on the right shows fibrous insulation 

discoloration caused by the filtering of particulates as air passes through it. 

Figure 3. Mold growing on the underside of roof sheathing (left) (DOE 2011); 

discoloration of insulation (right) 

(Left photo courtesy U.S. Department of Energy; right photo courtesy Efficiency Vermont)

These symptoms are indicative of poor air sealing or 

insulation levels (usually both). Both conditions can 

result in long-term energy performance and durability 

issues in the structure. To address these symptoms and 

to increase the long-term durability of the roof 

assembly, ensure proper air sealing and adequate 

insulation levels. Section 2.2 provides an in-depth 

discussion of the building science associated with these 

energy efficiency measures. 

2.2 Building Science in Energy Efficiency Measures

The need for roof repair often is a symptom of poor insulation performance, which, in turn, is 

symptomatic of air leakage. As air moves through or around the insulation, the heat or moisture 

it carries prematurely degrades all or part of the roof assembly. Signs of air leakage and poor 

insulation performance include wet, moldy, or rotting roof joists; discolored insulation; ice dams; 

snow or ice melting and freezing down the exterior walls of the building; water damage on the 

interior of the building; and the inability to maintain comfortable thermostat settings in some or 

all areas of the building. When making roof repairs, it is important to engage in preventive 

maintenance, which will reduce maintenance and energy costs. 

2.2.1 Air Seal

Air sealing is a preventive maintenance measure that saves in energy costs and roof maintenance. 

When a significant investment is being made in the roof, attending to its energy performance 

protects that investment. Occupant comfort also is increased greatly when a building is properly 

air sealed. Reducing air leakage can be much more difficult during retrofit than during new 

Building Science Principle –

Warm air has a greater capacity to 

carry moisture than cold air. When 

warm, moist air is allowed to migrate 

(leak) into a cold space, the moisture in 

the air condenses on the cold surface. 

This excess moisture eventually may 

lead to mold and rot, which degrade the 

structure.
5 

construction because many penetrations in existing buildings are covered with insulation or are 

inaccessible. Depending on the nature of the roof repair work, scheduling energy retrofit work 

concurrently may provide easier access to these areas.

The first step in creating an energy efficient roof structure is to define the thermal boundary, 

which separates conditioned (intentionally heated) space from unconditioned (unheated) space 

(see Figure 4). The thermal boundary (insulation) should be in direct contact with the air barrier, 

a continuous impermeable barrier that prevents undesirable air movement across the thermal 

boundary. 

Figure 4. The thermal boundary and air barrier separate the unconditioned space from the

conditioned space of a structure, preventing air infiltration indicated by the arrows (DOE 2011)

(Courtesy U.S. Department of Energy)

In Figure 4, the attic space is unconditioned. In buildings where the attic space is considered to 

be conditioned, the thermal boundary and air barrier would extend upward along the roof 

assembly. The thermal boundary must be continuous around the building envelope. 

Once the thermal boundary has been defined, all penetrations between the conditioned space and 

the unconditioned space should be sealed. These openings can include the following: 

• Plumbing, electrical, and mechanical chases, and other areas (e.g., attic hatches) open to 

the attic space.

• Gaps between the top plate of exterior walls and the rim or band joists. Figure 5

demonstrates the use of spray foam to effectively air seal and insulate this area.

• Recessed lighting fixtures.
6 

Figure 5. The air sealing benefits of spray foam make it a great option to insulate around rim or 

band joists 

(Photo courtesy Efficiency Vermont)

Any remaining air gaps can be identified by using a blower door in conjunction with an infrared 

camera or smoke pencil. Such diagnostic techniques also allow the effectiveness of air sealing 

efforts to be verified. Gaps should be filled with appropriate materials such as caulk, foam, or 

gaskets. Only fire-rated sealants should be used around chimneys, flues, and other fire-rated 

assemblies, designed to meet the requirements of the appropriate International Residential Code 

(IRC 2012, Chapter 3, Section R302) (IRC 2012) and ASTM standards (ASTM E84-12, ASTM 

E136-11, and ASTM E814-11a) (ASTM 2012, 2011a, and 2011b, respectively). 

Building codes are beginning to quantify maximum whole-building air leakage standards and are 

quickly becoming more aggressive in their requirements. The 2009 International Energy 

Conservation Code (IECC) includes an optional testing requirement of fewer than 7 air changes 

per hour at 50 Pascals of pressure (ACH50). This equates to roughly 1 cubic foot per minute 

(CFM) at 50 Pascals of pressure per square foot (CFM50/SF) of floor area. The 2012 IECC 

includes mandatory testing of air leakage and 

verification of no more than three ACH50 in climate 

zones 3 through 8 (DOE 2010). ENERGY STAR®

Version 3 requires fewer than four ACH50 for cold 

climates (approximately 0.50 CFM50/SF of floor 

area). A reasonable target for most existing 

multifamily buildings is an air leakage rate of no 

greater than 0.60 CFM50/SF of exterior surface area 

when performing a whole-building retrofit. These 

target values should be considered when air sealing 

the attic. Roof assemblies contribute largely to the 

overall air leakage in a building. Thorough air sealing 

efforts to provide high levels of airtightness are 

 Use a blower door during air sealing 

to be sure all major gaps have been 

adequately sealed.

 If a blower door test is not feasible, 

follow a comprehensive air sealing 

checklist such as the ENERGY STAR 

Thermal Enclosure Checklist (see 

Appendix A).

 Always conduct an air leakage 

inspection before installing insulation.
7 

particularly important to affordable housing projects to ensure long-term affordability to 

occupants.2

 

Links to detailed reports on specific air sealing strategies and air leakage testing for multifamily 

buildings, as well as examples and illustrations of air sealing methods related to roof repair or 

replacement, can be found in Steven Winter Associates (2010), Dentz et al. (2012), and 

Appendix A.

2.2.2 Insulate

Adding more insulation to an attic without first addressing air leakage can actually exacerbate 

the above-mentioned problems. For example, adding insulation to an attic with penetrations 

remaining unsealed does not prevent moist air from reaching the roof decking, but that surface is 

now more likely to be cold enough to cause condensation, which causes mold growth and other 

durability issues. 

Once the attic space has been air sealed, more insulation may be added to increase the thermal 

efficiency of the roof assembly. Improving the quantity and quality of insulation in the roof 

assembly is an investment that is repaid in energy savings and reduced maintenance. Attic 

insulation slows the transmission of heat through the roof. When a roof is covered in snow, 

excessive heat loss can be observed in patchy bare spots on the roof and ice dams forming at the 

eaves. When evaluating the attic space for installing insulation, pay attention to the amount of 

insulation to be installed and to how and where it is installed.

At minimum, current local code requirements (if applicable) must be met. Many state and 

municipal building codes are based on the IECC. In cold climates, the 2009 IECC requires a 

minimum R-38 in climate zone 5 and R-49 in climate zones 6 through 8. The 2012 IECC 

requires a minimum of R-49 in climate zones 5 through 8 (DOE 2010). ENERGY STAR 

recommends insulating attic spaces in the range of R-49 to R-60 in cold climates.3

 

As with air sealing, higher levels of insulation may be warranted in certain circumstances to 

achieve near- and long-term affordability (Shapiro 2011). For insulation to perform as it is rated, 

complete coverage and proper installation are critical. The goal when installing insulation is to 

achieve a Grade I installation per the Residential Energy Services Network (RESNET) Mortgage 

Industry National Home Energy Rating Standards (RESNET 2006). Grade I is defined most 

simply as insulation installed according to the manufacturer’s instructions. Poorly installed 

insulation (Grade II or Grade III) can reduce the efficiency of the thermal boundary at the roof 

assembly by as much as 40%.

 

2

A Roadmap for Housing Energy Affordability (Shapiro 2011) was sponsored by the MacArthur Foundation to 

address energy efficiency, building durability, and affordability needs in the Vermont affordable rental housing 

stock. Although this roadmap is directed to Vermont-specific needs, the basic lessons are applicable to any coldclimate multifamily housing. 

3 ENERGY STAR recommended insulation levels for retrofitting existing wood-framed buildings (ENERGY STAR

2011b).
8 

Although Grade I installation can be achieved with batt insulation, it is very difficult. Consider 

using a loose-fill insulation to reduce gaps and compression. When using loose-fill products, it is 

important to account for the settled density when 

specifying the final desired R-value. (R-value is a 

measure of thermal resistance, or how well a 

material prevents heat loss. It is the reciprocal of the 

U-value, which is the overall heat transfer 

coefficient that describes how well a building 

element conducts heat.) Complete coverage of 

insulation is as important as the overall quantity and 

quality of insulation. Even small gaps in insulation greatly affect the overall thermal efficiency 

of the entire assembly. Ideally, continuous insulation should cover the framing to prevent 

thermal bridging, which is the conductive transmission of heat through the framing structure. 

Attic areas that are commonly left 

uninsulated or underinsulated are 

access hatches and the intersections of 

roof rafters and exterior walls (over 

the wall top plate at the eaves). 

Reduced levels of insulation, even in 

relatively small areas of the attic, 

dramatically affect the overall R-value 

of the assembly. Where there are

sloped roof assemblies (the thermal 

boundary is at the roof deck), consider 

strategies such as rigid and spray foam 

products that allow for greater insulation values per inch. 

If the attic is unconditioned space, ventilation should also be assessed. Proper ventilation in the 

attic is important for moving excess moisture out of the building and maintaining the durability

of the roof. Almost all buildings with attic spaces will have a combination of soffit and gable 

vents, or a ridge vent and eave or soffit vents that run the entire length of the roof line. Some 

designers or builders will install insulation baffles (or rafter vents) into the roof joists to allow for 

ventilation from the eave upward to the ridge vent. In this design, the insulation contractor may 

need to install blocking to prevent the insulation from entering the eave but still allow for 

insulation all the way to the junction between the sloped roof and the eave. Follow local code 

requirements for proper attic ventilation. 

2.3 Risks and Other Considerations

When deciding whether to implement energy efficiency measures in a multifamily building, the 

following factors should be considered and then incorporated into the planning process: 

• Whole-building mechanical ventilation. When the roof assembly has been properly air 

sealed and insulated, it is critical to ensure that the occupied areas of the building are 

receiving adequate ventilation. Whole-building mechanical ventilation is essential to 

indoor air quality and building durability. Ventilation and controls should be installed to 

meet ASHRAE Standard 62.2 (ASHRAE 2010). ENERGY STAR-qualified products 

Is insulating the attic hatch really that important?

An uninsulated attic hatch can reduce the R-value of 

the entire assembly by 15%. Here’s how to calculate it:

Type R-Value

U-Value 

(1/R) Area

UA 

(Heat Loss)

(Area × U)

Ceiling 38 0.03 1990 52.37

Hatch 1 1.00 10 10.00

Total Area: Total UA:

2000 63.37

Area/UA = Average R-value

2000/63.67 = R-32

Grade I Insulation –

In practice, Grade I ceiling insulation fills 

the cavity uniformly and is in complete 

contact with the surface it is intended to 

insulate. There are no substantial gaps 

or voids around any obstructions such 

as electrical boxes or wiring. 
9 

should be used because they provide the same or better performance as standard products 

but use significantly less energy.

• Air distribution systems. Sealing ductwork found in attics or completely moving the 

ductwork into conditioned space could be considered to increase the distribution 

efficiency of a forced air system. 

• Blower door-assisted air sealing. Separately scheduling air sealing contractors and 

energy consultants to conduct blower door testing in combination with air sealing can be 

difficult. A certified contractor who is trained in the use of a blower door and who 

conducts blower door-assisted air sealing can reduce scheduling conflicts and ensure that 

the job is done right the first time.4 

• Nonflammable materials. Air sealing in multifamily buildings requires the use of 

nonflammable materials based on the fire separation assemblies in the building.

• Evaluate the space conditioning systems. As the building shell becomes significantly 

more energy efficient by reducing infiltration and adding insulation, heating and cooling 

loads are reduced. It is important to evaluate the space conditioning systems to ensure 

optimal performance and sizing of those systems.

• Occupant age. Elderly occupants typically require higher heating levels than do younger 

occupants. In these cases, higher thermal efficiency levels may be warranted.

• Historic preservation. In some cases, historic preservation guidelines may restrict or 

confine efficiency measures. Near- and long-term affordability may not be achievable 

under current historic preservation guidelines. 

• Financial assistance. Various state and federal incentives may be available for energy 

efficiency retrofit work. Review current resources that may be available. For example, 

information on state-level incentives is provided in the Database of State Incentives for 

Renewables & Efficiency (DSIRE 2011). Information on federal incentives is provided 

by the Tax Incentives Assistance Project (TIAP 2011).

2.4 Estimated Energy Savings

Estimated energy savings depend on many factors, making it difficult to provide average savings 

estimates. Savings will vary, depending on the size and baseline conditions of the building, the 

extent of the efficiency improvements, the efficiency of the heating equipment, the cost and type 

of fuel, and the climate location. With those caveats, some broad assumptions can be made, and a 

range of savings estimates can be provided as a guide for the impact of these efficiency 

measures. 

The following two examples provide estimated savings for completing air sealing and insulation 

work in 5-unit and 31-unit low-rise multifamily buildings. Both buildings are located in a cold 

climate (7,859 heating degree days at 65°F), and both buildings have flat ceilings and truss roof 

structures. Heating systems are assumed to be oil boilers having an 80% annual fuel utilization 

efficiency (AFUE). Air leakage has been reduced by 20%, and attic insulation has been increased 

from R-25 (a typical value for poorly installed existing attic insulation) to R-60. 

 

4 The Building Performance Institute provides individual professional certification and company accreditation for 

home performance and weatherization retrofit work standards (BPI 2011).
10 

2.4.1 Example 1: 5-Unit Town House Building

Air sealing penetrations into the attic space to reduce the overall infiltration rate from 

1.51 CFM50/SF exterior shell (10 ACH50) to 1.20 CFM50/SF exterior shell (8 ACH50) will save 

roughly 35 MMBtu annually. Bringing the total flat attic insulation from R-25 to R-60 will save 

an additional 14 MMBtu annually. Together the two measures provide a total building energy 

savings of approximately 50 MMBtu. Assuming the average annual consumption is 250 MMBtu 

(a rough average for this building type in a cold climate), this equates to roughly a 20% reduction 

in annual energy consumption. Installation and labor costs will vary widely by location but 

would likely be in the range of $10,000 to $15,000. Maintenance costs would also likely be 

reduced by improving the efficiency of the roof structure, and occupants would be more 

comfortable. 

2.4.2 Example 2: 31-Unit Double-Loaded Corridor

Air sealing penetrations into the attic space to reduce 

the overall infiltration rate from 2.07 CFM50/SF 

exterior shell (10 ACH50) to 1.66 CFM50/SF exterior 

shell (8 ACH50) will save roughly 136 MMBtu 

annually. Bringing the total flat attic area insulation 

from R-25 to R-60 will save an additional 33 MMBtu 

annually. Together the two measures provide a total 

building energy savings of about 170 MMBtu. 

Assuming the average annual consumption is 

2,000 MMBtu (a rough average for this building type 

in a cold climate), this equates to roughly an 8% 

reduction in annual energy consumption. Installation 

and labor costs will vary widely by location but 

would likely be $25,000 to $40,000. Maintenance 

costs would also likely be reduced by improving the 

efficiency of the roof structure, and the occupants would be more comfortable. 

 Greater savings will be realized in 

buildings with lower baseline 

conditions (more savings are 

captured from adding R-19 to an 

existing R-19 roof than by adding

R-19 to an existing R-38 roof).

 Consider the savings realized from 

energy efficiency improvements in 

comparison with fuel prices. It is 

important to think about life cycle 

benefits when evaluating the 

investment cost of energy efficiency 

improvements.
11 

3 Wall Repair or Gut Rehab

3.1 Energy Efficiency Opportunities

When the exterior walls of a building are in need of repair or a gut rehab, use the opportunity to 

consider energy efficiency measures that may be implemented concurrently with the scheduled 

repairs. In many cases, wall repairs are required because of underlying energy performance 

problems such as poor insulation and air sealing. Taking steps to create an energy efficient wall 

assembly will reduce building operating costs, increase occupant comfort, improve the durability 

of the exterior walls, and reduce long-term maintenance costs. Figure 6 shows excessive mold 

growth on interior walls and on the interior side of the exterior wall sheathing in a wood-framed 

wall assembly that was improperly air sealed and insulated. 

Figure 6. Mold growth on the inside surface of exterior walls (left) and inside the wall cavity (right). 

Both conditions are likely caused by an improperly air sealed and insulated wood-framed wall 

assembly. 

(BSC 2008) 

(Left photo courtesy Building Science Corporation)

To help guide where and what energy efficiency improvements might be incorporated into the 

exterior wall repairs, evaluate the condition of the wall insulation, exterior wall sheathing, air 

sealing, and exterior wall flashing details to assess what conditions may have contributed to the 

damage. Ask the following questions: 

• Is mold or rot present in the wall framing or sheathing?

• Is the insulation discolored? 

• Is there frost or condensation on the walls or glazing in winter? 
12 

• Has the exterior cladding been damaged? 

• Are any unsealed penetrations visually apparent through the wall assembly? 

• Is there any discoloration of the interior or exterior wall sheathing that would indicate 

water intrusion?

• Is the insulation completely filling the cavity space and in full contact with the air 

barrier?

Visual evidence of poor air sealing and insulation performance is shown in Figure 7. The image 

on the left shows mold growth on the interior walls as a result of poor air sealing at the floor. The 

image on the right shows fibrous insulation discoloration caused by the filtering of particulates as 

air passes through it. These symptoms are indicative of poor air sealing or insulation levels 

(usually both). To address these symptoms and to increase the long-term durability of the wall 

assembly, ensure proper air sealing and adequate insulation levels. 

Figure 7. Signs of poor air sealing and insulation performance

(BSC 2008, Housing Vermont 2010) 

(Left photo courtesy Building Science Corporation; right photo courtesy Efficiency Vermont)

The condition of the fenestration (doors and windows) should also be evaluated at this time. A 

lack of or damage to flashing material (such as sheet metal installed to protect the window 

framing and to direct moisture away from the opening) may have led to water entering the wall 

assembly, causing mold and other structural water damage. The wall assembly should be 

evaluated and addressed as a whole. Section 3.2 provides an in-depth discussion of the building 

science associated with these energy efficiency measures. 

3.2 Building Science in Energy Efficiency Measures

An excellent opportunity to address thermal and infiltration issues at the building core is 

presented when extensive repair or replacement of drywall on the interior side of exterior walls is 

required. Exterior walls typically are retrofitted for one of three reasons: 

1. Damage

2. Space reconfiguration

3. Energy efficiency upgrades. 
13 

This type of renovation allows for the removal of old, ineffective wall insulation or the addition 

of insulation if none was originally installed. Before replacing the insulation, examine the wall 

assemblies for air leakage and other factors that compromise the assemblies’ long-term 

durability. Improperly sealed walls allow moisture-laden air to move through and around 

insulation. This moisture condenses on cold surfaces, eventually leading to mold and 

deterioration of building assemblies such as wood-framed walls. Properly air sealing and 

replacing the insulation improves the thermal efficiency of the building and protects investments 

made to the building structure. 

3.2.1 Air Seal

In exterior wall repair or gut rehab projects, any 

insulation in the cavity needs to be removed and either 

reinstalled or replaced with new or higher quality 

product. Before reinsulating the wall cavity, the 

exterior sheathing will be exposed and the penetrations 

(including window and door openings) will be visible. 

This is a good opportunity to evaluate the air sealing 

details and the condition of the exterior wall that could 

have contributed to the damage and durability issues on 

the interior surface of the wall.

Proper air sealing before the insulation is replaced is 

critical. Air leakage affects occupant comfort by 

providing a path for cold air to leak into the building, 

allows heat to escape the building shell, and allows 

moisture to condense on the building. It also 

compromises the effectiveness of the space 

conditioning system. 

All penetrations through the exterior walls must be sealed at both the interior surface (drywall) 

and the exterior surface (sheathing) to prevent air leakage from both directions. Use a durable 

material such as caulk or foam. Acrylic latex or silicone caulk is suitable for gaps smaller than 

1⁄2 in. Higher grade silicone or polyurethane caulks should be used where the sealant will be 

exposed to weather. For large gaps and penetrations, a urethane-based spray foam is appropriate. 

Use minimally expanding spray foam around door and window rough openings to prevent 

warping of the frame. Use only fire-rated sealants around chimneys, flues, and other fire-rated 

assemblies, designed to meet the requirements of the appropriate International Residential Code 

(IRC 2012, Chapter 3, Section R302) (IRC 2012) and ASTM standards (ASTM E84-12, ASTM 

E136-11, and ASTM E814-11a) (ASTM 2012, 2011a, and 2011b, respectively). 

Penetrations through exterior walls and other conditions are likely to be in the following 

locations and would benefit from detailed air sealing efforts: 

• Plumbing or electrical penetrations

• Dryer duct or bath fan duct penetrations 

 Use a blower door during air 

sealing to be sure all major gaps 

have been adequately sealed.

 If a blower door test is not 

feasible, follow a comprehensive 

air sealing checklist such as the 

ENERGY STAR Thermal Bypass 

Checklist (see Appendix B).

 Always conduct an air leakage 

inspection before installing 

insulation.

 In buildings with balloon frame 

construction, air sealing between 

floors stops airflow and reduces 

fire hazards.
14 

• Behind tubs or showers located on exterior walls where insulation may be missing or 

where seams and access openings have old or cracked sealant (or none at all) 

• Bottom edge of sheetrock sealed to the subfloor on exterior walls 

• Around windows and door rough openings 

• Bulkheads located on exterior walls. 

The interior surface (drywall) is a key component of the thermal boundary system and air barrier. 

Once insulation has been reinstalled and the wall cavity has been enclosed, complete air sealing 

of the drywall is a critical detail. Any remaining air gaps can be identified by using a blower 

door in conjunction with an infrared camera or smoke pencil. Figure 8 shows examples of poor 

air sealing around doors and windows. In the photo on the left, fibrous insulation has been 

jammed into the gap above the door frame. Fibrous insulation is not an air barrier; it acts only as 

a filter and allows air to move freely between the interior and exterior. In the photo on the right, 

the gap between the window and the rough opening is not sealed at all. By contrast, Figure 9

shows two examples of good air sealing with low-expansion spray foam.

Figure 8. Poor air sealing around windows (ENERGY STAR 2011c) 

(Left photo courtesy Efficiency Vermont; right photo courtesy U.S. Department of Energy)
15 

Figure 9. (Left) Good air sealing around windows and (right) between drywall and subfloor

(Photos courtesy Efficiency Vermont)

Building codes are beginning to quantify maximum air leakage standards and are quickly 

becoming more aggressive in their requirements (see Section 2.2.1 for further discussion on 

building code air change requirements). A reasonable target for most existing multifamily 

buildings is an air leakage rate of no greater than 0.60 CFM50/SF of exterior surface area when 

performing a whole-building retrofit. These target values should be considered when air sealing 

the exterior walls. Infiltration can be reduced considerably by air sealing exterior walls. 

Thorough air sealing efforts to achieve high levels of airtightness are particularly important to 

affordable housing projects to provide near- and long-term affordability to occupants (Shapiro 

2011). Links to detailed reports on specific air sealing strategies and air leakage testing for 

multifamily buildings, as well as examples and illustrations of air sealing methods related to wall 

repair and gut rehab, can be found in Steven Winter Associates (2010), Dentz et al. (2012), and 

Appendix B. 

3.2.2 Fenestration Replacement or Repair

During wall repair or a gut rehab of the exterior walls, evaluate the condition of the fenestration 

(windows and doors). Determine if the current condition of the fenestration has contributed to 

damage of the exterior wall assembly. Assess the flashing, integration with the drainage plane, 

and air sealing around the fenestration. Is moisture damage visible on the wall around the 

windows? What is the condition of the thermal break in the window frames? Is condensation 

visible on the walls or between glazing units? 

When it is determined that windows need to be replaced because of excessive damage or 

operability issues, energy savings opportunities may be addressed at the same time by specifying 

windows with better performance values. At minimum, select windows that meet local code 

requirements. For increased efficiency, consider upgrading to ENERGY STAR-qualified

windows. Replacing windows for energy savings alone typically is not cost effective. 

If only slight modifications or repairs are required, restoring existing windows may be a more 

economical approach. A number of actions may be considered that can improve the energy 

efficiency of the windows while executing the restoration: 
16 

• Check for loose glass and failing casing, rails, sash, or muttons, and repair as necessary. 

• For double-hung windows, repair or install new sash locks at the junction between the 

upper and lower rails as well as on the left and right sides of the window jam. 

• Install weather stripping at the bottom sill to provide additional air sealing and ensure that 

flashing remains effective. Bulk moisture should drain away from building materials (the 

job of window flashing), and interior moisture should not have an opportunity to 

condense on building materials (the job of air sealing). Remember that air sealing done 

incorrectly or in the wrong place (say, the exterior instead of the interior) and improper 

flashing can create further problems such as trapping water inside the wall assembly. 

• Add interior and exterior storms when possible to improve window performance. 

• Properly retrofit single-pane windows (these can perform as well as standard double-pane 

replacement windows). 

There are many variables to consider when evaluating the best strategy for window repair, 

rehabilitation, or replacement, and attention should be paid to flashing details to preserve 

window and building wall durability (Baker 2012; BSC 2012). 

3.2.3 Insulate

Once the exterior walls have been adequately air sealed, insulation can be installed. Improving 

the thermal efficiency of the exterior wall assembly is an investment that is repaid in reduced 

operating and maintenance costs. However, reinsulating or adding insulation to a wall assembly 

that was previously uninsulated will change the dynamics of the building enclosure. For 

example, if the original condition was a leaky wall 

assembly with no insulation, the exterior sheathing 

may have seen moisture from indoor air, but the 

sheathing may have remained warm enough (from

lack of insulation) so that condensation did not occur. 

If insulation is now properly installed but not properly

air sealed, air can still leak into the cavity, introducing 

moisture into the assembly. Insulated walls will 

thermally isolate the exterior sheathing from the conditioned space. In this condition, there is a 

greater potential for condensation and durability issues. Therefore, it is critical to thoroughly 

evaluate the details of the entire wall assembly and retrofit measures. 

At minimum, insulate exterior walls to current local code standards (if applicable). In cold 

climates, the 2009 IECC requires a minimum of R-20 in climate zones 5 and 6. R-21 is required 

in climate zones 7 and 8. The 2012 IECC requires a minimum of R-20 in climate zone 5. Climate 

zones 6 through 8 require a minimum of R-20 cavity insulation plus R-5 continuous insulation 

(or R-13 cavity plus R-10 continuous) (DOE 2010). Shapiro (2011) recommends exterior wall 

insulation ranging from code levels up to R-36 for very poorly performing multifamily structures 

to meet near- and long-term affordability goals. For insulation to perform as it is rated, complete 

coverage and proper installation are critical. As with attic insulation, the goal when installing 

wall insulation is to achieve a Grade I installation per the RESNET Standards (RESNET 2006). 

Grade I Insulation –

In practice, Grade I insulation fills the 

cavity uniformly and is in complete 

contact with the surface it is intended to 

insulate. There are no substantial gaps 

or voids around any obstructions such 

as electrical boxes or wiring. 
17 

Poorly installed wall insulation (Grade II or 

Grade III) can reduce the thermal efficiency of the 

wall assembly by as much as 20%.

Consider alternative cavity insulation options such 

as dense-pack cellulose or two-part spray foam 

products that provide air sealing benefits in addition 

to insulation. In standard construction, nearly 20% 

of the wall system is framing, not insulation. 

Thermal bridging occurs through the framing studs 

that are not covered with insulation because heat 

transfers easily through uninsulated stud framing to 

the outdoors. This has a large impact on the overall R-value of the wall assembly. Identifying 

strategies that can reduce the overall impact of the framing can provide significant benefits to the 

overall performance of the assembly and the energy efficiency of the building. Continuous rigid 

insulation on the inside or outside of exterior walls reduces thermal bridging through the wall 

framing and is especially critical in steel-framed structures. 

Figure 10 shows examples of proper installation of rigid foam, which may be installed on the 

interiors or exteriors of walls. Cap nails should be used, and all seams should be sealed 

completely. 

Figure 10. Proper installation of continuous rigid foam (ENERGY STAR 2011c), showing the use of 

cap nails and proper sealing of seams

(Photos courtesy U.S. Department of Energy)

3.3 Risks and Other Considerations

As with repairing or replacing the roof, risks and other factors should be considered and 

incorporated into the planning process when undertaking any major shell improvement (see 

Section 2.3 for more information).

Effective R-Values –

An R-21 fiberglass batt does not 

equate to an R-21 wall assembly. 

Cavity insulation in a 2 × 6 wall rated at 

R-21 typically equates to a wall 

assembly R-value ranging from R-18 

(Grade I) to R-14 (Grade III), 

depending on the type of framing and 

the quality of the insulation installation. 

The better the installation quality, the 

higher the effective R-value will be.
18 

3.4 Estimated Energy Savings

Estimated energy savings depend on many factors, making it difficult to provide average savings 

estimates. Savings will vary, depending on the size 

and baseline conditions of the building, the degree 

of the efficiency improvements, the efficiency of 

the heating equipment, the cost and type of fuel, 

and the climate location. With those caveats, some 

broad assumptions can be made, and a range of 

savings estimates can be provided as a guide for the 

impact of these efficiency measures. 

Examples of energy savings are again provided 

based on a 5-unit and a 31-unit low-rise 

multifamily building in a cold climate. The wall 

system of both buildings is 2 × 6 wood studs with a 

brick façade. Heating systems are assumed to be oil boilers having an 80% AFUE. Air leakage 

has been reduced by 10%, and wall insulation has been increased from R-13 to R-20. Savings 

from window replacements assume single-pane with storm windows or double-pane wood-frame 

(U-factor 0.49) baseline windows upgraded to ENERGY STAR-qualified (U-factor 0.30) 

windows. 

3.4.1 Example 1: 5-Unit Town House Building

Air sealing penetrations through the exterior walls to reduce the overall infiltration rate from 

1.51 CFM50/SF exterior shell (10 ACH50) to 1.35 CFM50/SF exterior shell (9 ACH50) will save 

roughly 17 MMBtu annually. Bringing the exterior wall insulation from R-13 to R-20 will save 

an additional 24 MMBtu annually. Together the two measures provide a total build energy 

savings of approximately 41 MMBtu. Assuming the average annual consumption is 250 MMBtu 

(a rough average for this building type in a cold climate), this equates to approximately a 16% 

reduction in annual energy consumption. Installation and labor costs will vary widely by location 

but would likely be $10,000 to $15,000. In addition to the annual energy cost savings, 

maintenance costs would likely be reduced by improving the efficiency of the wall assembly, 

and occupants would be more comfortable. If windows also are being replaced, an additional 

26 MMBtu may be saved annually. Costs associated with the installation of new ENERGY 

STAR-qualified windows may range from $30,000 to more than $60,000. The cost of window 

restoration can run approximately 75% less than replacement costs; similar levels of energy 

savings would be realized. 

3.4.2 Example 2: 31-Unit Double-Loaded Corridor

Air sealing penetrations through the exterior walls to reduce the overall infiltration rate from 

2.07 CFM50/SF exterior shell (10 ACH50) to 1.87 CFM50/SF exterior shell (9 ACH50) will save 

roughly 68 MMBtu annually. Bringing the exterior wall insulation from R-13 to R-20 will save 

an additional 62 MMBtu annually. Together the two measures provide a total building energy 

savings of about 130 MMBtu. Assuming the average annual consumption is 2,000 MMBtu (a 

rough average for this building type in a cold climate), this equates to approximately a 7% 

reduction in annual energy consumption. Installation and labor costs will vary widely by location 

but would likely be $25,000 to $40,000. In addition to annual energy cost savings, maintenance 

costs would likely be reduced by improving the efficiency of the wall assembly, and occupants 

 Greater energy savings will be 

realized in buildings with poorerperforming conditions.

 Consider the savings realized from 

energy efficiency improvements in 

comparison with fuel prices. It is 

important to think about life cycle 

benefits when evaluating the 

investment cost of energy 

efficiency improvements.
19 

would be more comfortable. If windows also are being replaced, an additional 67 MMBtu may 

be saved annually. Costs associated with the installation of new ENERGY STAR-qualified

windows may range from $90,000 to more than $150,000. The cost of window restoration can 

run approximately 75% less than replacement costs; similar levels of energy savings would be 

realized. 
20 

4 Boiler Maintenance and Retrofit

4.1 Energy Efficiency Opportunities

The renovation of an existing building is an opportune time to assess the condition of the heating 

system. Whether for space reconfiguration or improvements to the enclosure, renovation 

provides an excellent opportunity to upgrade the heating system. An evaluation of the existing 

heating unit should include the age of the system. When the heating system is 20 years old or 

older, replacement is recommended. Additionally, if the existing heating system is inefficient or 

performs poorly, replacement should be considered. If the heating system is going to be replaced, 

the building heat load should be recalculated. When renovations include improvements to the 

building shell, the design loads of the building can change significantly. For this reason, it is 

important to improve the performance of the shell before considering replacement of heating 

systems. Oversizing heating systems, particularly oil-fired or condensing equipment, will 

effectively degrade efficiencies. Note that the building airtightness (air changes per hour) is a 

critical factor in the building load calculation. If possible, a blower door test should be performed 

to determine actual building leakage. Once the design load is known, the next important 

consideration is the new unit’s rated thermal efficiency (provided as the AFUE). Select a boiler 

that, at minimum, meets ENERGY STAR boiler requirements. The boiler also should include the 

controls as discussed below. 

Figure 11 shows a typical heating unit for a multifamily building. With such a system, a lack of 

microprocessor controls provides a good opportunity for retrofit.

Figure 11. Typical multifamily heating unit without microprocessor controls (Klein 2011)

(Photo courtesy Gary Klein, Affiliated International Management, LLC)

Regular heating system maintenance and service are important components of the operation and 

maintenance functions of a building. The manufacturer guidebook recommendations for regular 
21 

maintenance should be followed and will include cleaning combustion chambers, heat exchange 

surfaces, and nozzles; ensuring adequate combustion air intake; checking for sufficient venting; 

replacing filters; and performing combustion efficiency tests. System maintenance extends the 

life of the equipment and prevents gas leaks (from fittings, unions, and valves); carbon monoxide 

poisoning (from cracked furnace heat exchangers, faulty flues, or backdrafting); and fires (from 

faulty controls or faulty wiring). It also helps to keep the system operating at its rated efficiency 

level. With regular maintenance and service, a boiler can remain in operation and perform 

effectively for approximately 20 years. 

While maintaining an existing system, some retrofit measures or upgrades should be considered 

to further improve its overall efficiency. Retrofit opportunities exist to improve the overall 

efficiency of the heating system by adding microprocessor controls to existing boilers. Many 

types of controls are available, ranging from simple to full direct digital controls. These controls 

provide the ability to adjust the output of the boiler based on external conditions and demand. 

The building owner should select controls based on the size and age of the boiler system and the 

facility personnel’s level of comfort with the controls.

When a boiler is scheduled for routine maintenance 

or repair, the following should be considered to 

increase its energy efficiency and performance:

• Outdoor reset controls 

• Domestic hot water priority

• Brushless permanent magnet (BLPM) 

circulator pumps with integrated variable 

frequency drive 

• Staging controls for multiple boilers

• Warm-weather shutdown controls 

• Building-wide thermostat setback controls. 

4.2 Building Science in Energy Efficiency Measures

Boiler microprocessor controls can be added and programmed to improve the overall efficiency 

of the heating system. The following controls, which could reside within a single device, are 

recommended for retrofits: 

• Outdoor reset controls are used to modulate boiler water temperature in inverse 

proportion to the outdoor temperature, thereby minimizing the potential for overheating 

and wasted heat. 

• Domestic hot water priority is for systems with direct hot water storage tanks. This 

control will reset the boiler water temperature to 180°F during the period of time the hot 

water tank is calling for heat. Once the call for heat ends, the boiler water temperature 

will reset to the temperature calculated by the outdoor reset controller. The result is the 

 Require system commissioning to ensure 

proper installation and performance 

testing.

 In all units, use programmable 

thermostats protected with lock boxes in 

common areas or remotely located. 

 Models that have easy-to-read displays 

and that can provide intuitive adjustments 

are particularly important for elderly 

occupants. 

 All occupants should be provided with 

instructions on the use of new 

thermostats.
22 

ability to meet domestic hot water needs without keeping boiler water temperatures hotter 

than necessary to meet space heating needs.

• Pump demand controls allow a variable flow heating loop. Upgrade to a loop pump with 

a BLPM motor and an integrated variable frequency drive with a built-in pressure sensor.

Circulator pumps that use BLPMs are more efficient because they lack brushes that add 

friction to the motor and can modulate their speed to match the load. Pumping energy 

then is minimized while meeting needs. 

• Boiler staging lead/lag controls are used when multiple boilers are present to provide 

only as much boiler capacity as is needed. Controls are programmed so that the lead 

boiler alternates to ensure uniform “wear and tear” of the boilers. As with the other 

recommended controls, wasted heat as a result of excess heating is minimized for 

important savings without sacrificing the ability to meet legitimate needs.

• Warm-weather shutdown controls also may be incorporated as a fail-safe mechanism to 

ensure boilers are not firing when the outside temperature rises above a preset outdoor 

temperature (60°–65°F is recommended). This fail-safe control ensures that reset control 

strategies are further optimized for additional savings.

• Building-wide thermostat setback controls allow the building owner to set back the 

thermostat temperature settings in the common halls and the tenant units. This control 

sequence would require that thermostats and boiler controls can “talk” to each other. This 

strategy helps to minimize overheating during periods of typical inactivity such as the 

early morning hours when comfort zone temperatures are not needed. Common halls also 

may be kept cooler than tenant units for additional savings. 

Figure 12 shows a replacement boiler with integrated processor controls and associated water 

flow control. 
23 

Figure 12. Replacement boiler with microprocessor controls (left); 

associated water flow control (right) 

(Housing Vermont 2010) 

(Photos courtesy Efficiency Vermont)

4.3 Risks and Other Considerations

When a heating system is 20 years old or older, it 

should be replaced with ENERGY STARqualified units that have integrated controls (as 

described previously). Oil boilers should have an 

AFUE rating of 85% or greater. Propane and 

natural gas boilers should have an AFUE of 90% 

or greater. ENERGY STAR boilers use

approximately 10% less energy than boilers that 

meet federal minimum standard efficiency levels

(ENERGY STAR 2011a). If replacing the boiler, 

consider installing a modulating output boiler. 

Modulating boilers can match exactly the 

building load by adjusting the burn rate, which 

allows for low-load operation with no sacrifice in 

fuel efficiency. Additional guidance on boiler 

maintenance and retrofitting for multifamily 

buildings can be found in Dentz and Henderson 

(2012), Choi et al. (2013), and Appendix C. 

Proper Sizing of Heating Systems –

When installing new heating equipment, ensure that 

proper sizing calculations are performed and that 

equipment selection guidelines are followed. 

Oversized heating systems are energy inefficient 

and can reduce occupant comfort, indoor air quality, 

and the overall durability and performance of the 

equipment. Oversized equipment short-cycles, 

meaning the equipment turns on and off more 

frequently than it was designed to do. This increased 

cycling leads to greater rates of equipment failure. 

Additionally, the system does not perform at its rated 

efficiency level. Comfort issues arise when the 

heating system does not stay on long enough to 

uniformly heat a space.

Performing updated system sizing calculations is 

critical after improvements are made to the building 

shell. Guidance on accurately calculating heating 

and cooling loads can be found in Rutkowski (1995 

and 2006) and in Appendix C.
24 

If the boiler system is going to be replaced, it is critical to reevaluate the heating and cooling 

demands of the building because other improvement and retrofit activities could have changed 

the load profile and conditioning needs throughout the space. There is also no guarantee that the 

original equipment was sized and selected accurately. Load calculations and equipment sizing 

and selection should be completed in accordance with Air Conditioning Contractors of America 

Manual J and Manual S (Rutkowski 2006 and 1995, respectively). 

If the building has a steam boiler that is 15 years old or older, significant savings could be 

realized by converting it to a hot water boiler. One study reported savings of more than 60% 

from converting an old steam-heating system to a hydronic distribution system including high 

efficiency condensing boilers, indirect-fired domestic hot water, and zone valves (Nolden 1995). 

In some buildings, the heating demand in different zones cannot be sufficiently controlled. This 

can result in building operators overheating some units to ensure that other units receive enough 

heat. In these circumstances, the most expensive option would be to add zone valves and new 

wall thermostats to each unit as described above. However, a lower cost option would be to 

install thermostatic radiator valves that regulate the flow of hot water to the radiator based on the 

temperature in the room and the occupant’s desired set point. 

4.4 Estimated Savings

The estimated savings for retrofitting boilers will vary greatly, depending on the heating system 

and what controls are applied. The savings ranges provided here are based on various studies and 

analyses showing achievable savings when these types of controls are installed.

Examples of energy savings are again provided based on a 5-unit and a 31-unit low-rise 

multifamily building. The roof and wall improvements discussed are assumed to have been 

made, thereby lowering the heat load of the building. The savings from retrofitting controls and 

pumps assume that the heating systems are oil boilers with an 80% AFUE. 

• Adding outdoor reset controls to a single boiler can save approximately 5% in annual 

heating energy consumption (DTE Energy 2011a and 2011b). This would amount to 

approximately 8–12 MMBtu in energy savings for the 5-unit building and 25–30 MMBtu 

for the 31-unit building. A typical install cost is roughly $1,000. 

• Adding boiler staging and outdoor reset controls typically saves about 10% in annual 

heating energy consumption (DTE Energy 2011a and 2011b). This would amount to 

approximately 16–24 MMBtu in energy savings for the 5-unit building and 50–

60 MMBtu for the 31-unit building. A typical install cost is roughly $1,500. 

• Upgrading to a BLPM circulator pump with integrated pump demand controls can 

provide savings ranging from 400 kWh/yr for small applications such as the 5-unit 

building (less than 1.25 amps) to more than 2,000 kWh/yr for larger applications such as 

the 31-unit building (more than 5 amps), assuming the primary loop circulator runs 

constantly during the heating season. On average, this represents a savings of 70% to 

80% over standard circulator pumps (VEIC 2011). Typical install costs range from 

$1,000 to $2,000, depending on the size of the motor and local labor rates.

• A number of effective thermostatic control devices are available to assist building 

operators and tenants with advanced scheduling of temperature setbacks (or setup for 
25 

cooling applications). The potential for savings can be significant, although the 

magnitude and persistence of the savings rely on appropriate behavior and effective 

programming of the devices. Installing five programmable thermostats in the 5-unit 

building would cost approximately $250 to $500, depending on the sophistication of the 

thermostats and could save 8–12 MMBtu. In the 31-unit building, more advanced 

thermostat and central remote management of heating, ventilation, and air-conditioning 

controls could be considered, which could cost $3,000 to $5,000 and could provide an 

additional 50–100 MMBtu in heating savings and 500–1,000 kWh in cooling savings. 
26 

5 Retrofit Measure Strategy Guideline Overview

Figure 13 through Figure 15 provide an overview of each common building retrofit measure. 

Tied to each retrofit measure are the recommended energy efficiency measures that could be 

implemented at the time of retrofit, as well as important points to keep in mind when performing 

these retrofits. Potential energy savings ranges are provided for each energy efficiency retrofit 

measure.
27 

Figure 13. Roof repair or replacement retrofit measure

Figure 14. Wall repair or gut rehab retrofit measure

Figure 15. Boiler maintenance and retrofit measure
28 

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Buildings. ANSI/ASHRAE Standard 62.2-2010. Atlanta, GA: ASHRAE.

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Tube Furnace at 750°C. West Conshohocken, PA: ASTM International. 

www.astm.org/Standards/E136.htm. 

ASTM. (2011b). ASTM E814-11a Standard Test Method for Fire Tests of Penetration Firestop 

Systems. West Conshohocken, PA: ASTM International. www.astm.org/Standards/E814.htm. 

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14%20BSC%20Window%20Repair,%20Rehab,%20and%20Replace.pdf. 

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MA: Building Science Corporation. Accessed May 8, 2012: 

www.buildingscience.com/documents/insights/bsi-004-drainage-holes-and-moderation. 

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for Multifamily Residential Buildings. NREL Subcontractor Report: SR-5500-57385. Golden, 

CO: National Renewable Energy Laboratory. 

Dentz, J.; Conlin, F.; Podorson, D. (2012). Case Study of Envelope Sealing in Existing Multiunit 

Structures. NREL Subcontractor Report: SR-5500-54787. Golden, CO: National Renewable 

Energy Laboratory. 
29 

Dentz, J.; Henderson, H. (2012). Hydronic Heating Retrofits for Low-Rise Multifamily 

Buildings—Phase 1: Boiler Control Replacement and Monitoring. NREL Subcontractor Report: 

SR-5500-53806. Golden, CO: National Renewable Energy Laboratory. 

DOE. (2010). Guide to Determining Climate Regions by County. Building America Best 

Practice Series, Volume 7.1. August 2010. Washington, DC: U.S. Department of Energy. 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ba_climateguide_7_1.

pdf. 

DOE. (2011). House as a System—Affordable Comfort 2011. Washington, DC: U.S. 

Department of Energy, Energy Efficiency & Renewable Energy. Accessed February 1, 2012: 

http://2011.acinational.org/sites/default/files/session/80948/aci11fund1coxanthony.pdf. 

DSIRE. (2011). Database of State Incentives for Renewables & Efficiency. Accessed January 6, 

2012: www.dsireusa.org/. 

DTE. (2011a). Heat-Timer—Outdoor Reset Control Savings. Detroit, MI: DTE Energy. 

Accessed January 11, 2012: www.heat-timer.com/en/EducationDetail.aspx?Id=3. 

DTE. (2011b). Your Energy Savings Business Program–Boiler Reset Controls. Detroit, MI: DTE 

Energy. Accessed January 11, 2012: www.dteenergy.com/pdfs/boilerResetFactSheet.pdf. 

ENERGY STAR. (2011a). How Efficient Are ENERGY STAR-qualified Boilers? Accessed 

January 6, 2012: 

http://energystar.supportportal.com/ics/support/kbAnswer.asp?deptID=23018&task=knowledge

&questionID=17331. 

ENERGY STAR. (2011b). Recommended Levels of Insulation. Accessed January 6, 2012: 

www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table. 

ENERGY STAR. (2011c). Thermal Enclosure System Rater Checklist. Accessed February 1, 

2012: 

www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Energy_Star_v3_TERC_Guide

book.pdf. 

Housing Vermont. (2010). Burlington, VT: Housing Vermont. Accessed February 1, 2012: 

http://hvt.org/proddir/search/237/?Search[county]=Addison. 

IRC. (2012). International Residential Code for One- and Two-Family Dwellings. Country Club 

Hills, IL: International Code Council. Chapter 3, Section R302, Fire-Resistant Construction.

Klein, G. (2011). Practical Plumbing for Multifamily Buildings. Newport Beach, CA: Affiliated 

International Management, LLC.

Nolden, S. (1995). The Key to Persistence. Home Energy Magazine Online, September/October 

1995. www.homeenergy.org/show/article/nav/heating/page/6/id/1157. 
30 

RESNET. (2006). 2006 Mortgage Industry National Home Energy Rating Systems Standards. 

Oceanside, CA: Residential Energy Services Network. Accessed January 6, 2012: 

www.resnet.us/standards/ RESNET_Mortgage_Industry_National_HERS_Standards.pdf.

Rutkowski, H. (1995). Manual S—Residential Equipment Selection. Arlington, VA: Air 

Conditioning Contractors of America. 

Rutkowski, H. (2006). Manual J—Residential Load Calculation, 8th edition, Version 2. 

Arlington, VA: Air Conditioning Contractors of America. 

Shapiro, A.M. (2011). A Roadmap for Housing Energy Affordability. Burlington, VT: Vermont 

Housing Finance Agency, March 2011. Accessed January 6, 2012: www.vhcb.org/roadmap.pdf. 

Steven Winter Associates. (2010). Air Sealing Guide: Multifamily Construction. Norwalk, CT: 

Steven Winter Associates. http://carb-swa.com/articles/guidelines/MultiFamily%20Air%20Sealing%20Guide.pdf. 

TIAP. (2011). The Tax Incentives Assistance Project. Accessed January 6, 2012: 

www.energytaxincentives.org/. 

VEIC. (2011). Burlington, VT: Vermont Energy Investment Corporation. Accessed January 6, 

2012: www.veic.org. 
31 

Appendix A: Resources for Roof Repair or Replacement

Building America. Retrofit Techniques and Technologies: Air Sealing—A Guide for Contractors 

to Share with Homeowners. Building America Best Practice Series. Accessed January 6, 2012: 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ba_airsealing_report.

pdf. 

Building Science. Guide to Attic Air Sealing. Somersville, MA: Building Science Corporation. 

Accessed January 6, 2012: www.buildingscience.com/documents/guides-and-manuals/gm-atticair-sealing-guide. 

Dentz, J.; Conlin, F.; Podorson, D. (2012). Case Study of Envelope Sealing in Existing Multiunit 

Structures. NREL Subcontractor Report: SR-5500-54787. Golden, CO: National Renewable 

Energy Laboratory. 

ENERGY STAR. ENERGY STAR for Homes Version 3 Guidelines. Accessed January 11, 

2012: www.energystar.gov/index.cfm?c=bldrs_lenders_raters.nh_v3_guidelines. 

ENERGY STAR. ENERGY STAR Thermal Enclosure System Rater Checklist. Accessed 

January 6, 2012: 

www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Energy_Star_v3_TERC_Guide

book.pdf. 

Hynek, Don. Blower Door Testing in Multifamily Buildings. Home Energy Magazine, 

August 31, 2011. Accessed January 6, 2012: 

www.homeenergy.org/show/article/nav/auditing/id/1711. 

Otis, C.; Maxwell, S. (2012). Measure Guideline: Air Sealing Attics in Multifamily Buildings. 

NREL Subcontractor Report: SR-5500-54720. Golden, CO: National Renewable Energy 

Laboratory. 

Shapiro, A.M. (2011). A Roadmap for Housing Energy Affordability. Burlington, VT: Vermont 

Housing Finance Agency, March 2011. Accessed January 6, 2012: www.vhcb.org/roadmap.pdf. 

Steven Winter Associates. (2010). Air Sealing Guide: Multifamily Construction. Norwalk, CT: 

Steven Winter Associates. http://carb-swa.com/articles/guidelines/MultiFamily%20Air%20Sealing%20Guide.pdf. 
32 

Appendix B: Resources for Wall Repair or Gut Rehab

Baker, P. (2012). Measure Guideline: Window Repair, Rehabilitation, and Replacement. NREL 

Subcontractor Report: SR-5500-55219. Golden, CO: National Renewable Energy Laboratory. 

Also presented at Building America Experts Meeting, November 14, 2011, Minneapolis, MN. 

Somerville, MA: Building Science Corporation. 

www.buildingscienceconsulting.com/presentations/documents/2011-11-

14%20BSC%20Window%20Repair,%20Rehab,%20and%20Replace.pdf. 

Dentz, J.; Conlin, F.; Podorson, D. (2012). Case Study of Envelope Sealing in Existing Multiunit 

Structures. NREL Subcontractor Report: SR-5500-54787. Golden, CO: National Renewable 

Energy Laboratory. 

ENERGY STAR. ENERGY STAR for Homes Version 3 Guidelines. Accessed January 11, 

2012: www.energystar.gov/index.cfm?c=bldrs_lenders_raters.nh_v3_guidelines. 

ENERGY STAR. ENERGY STAR Thermal Enclosure System Rater Checklist. Accessed 

January 6, 2012: 

www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Energy_Star_v3_TERC_Guide

book.pdf. 

Shapiro, A.M. (2011). A Roadmap for Housing Energy Affordability. Burlington, VT: Vermont 

Housing Finance Agency, March 2011. Accessed January 6, 2012: www.vhcb.org/roadmap.pdf. 

Steven Winter Associates. (2010). Air Sealing Guide: Multifamily Construction. Norwalk, CT: 

Steven Winter Associates. http://carb-swa.com/articles/guidelines/MultiFamily%20Air%20Sealing%20Guide.pdf. 
33 

Appendix C: Resources for Boiler Maintenance and Retrofit

Burdick, A. Strategy Guideline: Accurate Heating and Cooling Load Calculations. Golden, CO: 

National Renewable Energy Laboratory. Accessed January 6, 2012: 

www.nrel.gov/docs/fy11osti/51603.pdf. 

Choi, J.; Ludwig, P.; Brand, L. (2013). Measure Guideline: Steam System Balancing and Tuning 

for Multifamily Residential Buildings. NREL Subcontractor Report: SR-5500-57385. Golden, 

CO: National Renewable Energy Laboratory. 

Dentz, J.; Henderson, H. (2012). Hydronic Heating Retrofits for Low-Rise Multifamily 

Buildings—Phase 1: Boiler Control Replacement and Monitoring. NREL Subcontractor Report: 

SR-5500-53806. Golden, CO: National Renewable Energy Laboratory. 

DTE Energy (2011a). Heat-Timer—Outdoor Reset Control Savings. Detroit, MI: DTE Energy. 

Accessed January 11, 2012: www.heat-timer.com/en/EducationDetail.aspx?Id=3. 

DTE Energy (2011b). Your Energy Savings Business Program—Boiler Reset Controls. Detroit, 

MI: DTE Energy. Accessed January 11, 2012: 

www.dteenergy.com/pdfs/boilerResetFactSheet.pdf. 

Nolden, S. (1995). The Key to Persistence. Home Energy Magazine Online, September/October 

1995. www.homeenergy.org/show/article/nav/heating/page/6/id/1157. 
DOE/GO-102013-3838 ▪ April 2013

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